WHAT IS A PLATE? MAJOR PLATES. Types of Earth’s Crust. Plate Boundary

DM 5113, MSDM, DU

1. PLATE TECTONICS
a)What Is A Plate?
b)Plate Boundary
c)Plate Tectonics Theory
d)Continental Drift
e)Sea Floor Spreading
f)Driving Forces of Plate Motion
g)Types of plate boundaries
h)Significance of Plate Tectonics

2. •The lithosphere is broken up into large segments what are called
tectonic plates. Tectonic plates consist of lithospheric mantle (upper
part of the upper mantle) overlain by either of two types of crustal
material: oceanic crust (in older texts called sima from silicon and
magnesium) and continental crust (sial from silicon and aluminium).
Average oceanic lithosphere is typically 100 km thick; its thickness is a
function of its age: as time passes, it conductively cools and becomes
thicker. Continental lithosphere is typically ~200 km thick, though this
also varies considerably between basins, mountain ranges, and stable
cratonic interiors of continents. The two types of crust also differ in
thickness, with continental crust being considerably thicker than
oceanic (35 km vs. 6 km).
•Major continental and oceanic plates include:
the Eurasian plate, Australian-Indian plate, Philippine plate, Pacific
plate, Juan de Fuca plate, Nazca plate, Cocos plate, North American
plate, Caribbean plate, South American plate, African plate, Arabian
plate, the Antarctic plate, and the Scotia plate. These plates consist of
smaller sub-plates.
WHAT IS A PLATE?

3. MAJOR PLATES

4. Types of Earth’s Crust
•Tectonic plates can include continental crust or oceanic crust, and
many plates contain both. For example, the African Plate includes the
continent and parts of the floor of the Atlantic and Indian Oceans. The
distinction between oceanic crust and continental crust is based on
their modes of formation. Oceanic crust is formed at sea-floor
spreading centers, and continental crust is formed through
arc volcanism and accretion of terranes through tectonic processes;
though some of these terranes may contain ophiolite sequences, which
are pieces of oceanic crust, these are considered part of the continent
when they exit the standard cycle of formation and spreading centers
and subduction beneath continents. Oceanic crust is also denser than
continental crust owing to their different compositions. Oceanic crust is
denser because it has less silicon and more heavier elements ("mafic")
than continental crust ("felsic"). As a result of this density stratification,
oceanic crust generally lies below sea level (for example most of the
Pacific Plate), while the continental crust buoyantly projects above sea
level (see isostasy for explanation of this principle).

5. Types of Crust

6. Plate Boundary
•What is a plate boundary?
•The location where two plates meet is called a plate
boundary, and plate boundaries are commonly associated
with geological events such as earthquakes and the creation
of topographic features such as mountains, volcanoes,
mid-ocean ridges, and oceanic trenches. The majority of the
world's active volcanoes occur along plate boundaries, with
the Pacific Plate's Ring of Fire being most active and most
widely known.
•Tectonic plates can include continental crust or oceanic crust,
and many plates contain both. For example, the African Plate
includes the continent and parts of the floor of the Atlantic
and Indian Oceans. The distinction between oceanic crust and
continental crust is based on their modes of formation.
Oceanic crust is formed at sea-floor spreading centers, and
continental crust is formed through arc volcanism and
accretion of terranes through tectonic processes .

7. Key Principles
•The outer layers of the Earth are divided into lithosphere and
asthenosphere. This is based on differences in mechanical
properties and in the method for the transfer of heat.
Mechanically, the lithosphere is cooler and more rigid, while the
asthenosphere is hotter and flows more easily. In terms of heat
transfer, the lithosphere loses heat by conduction whereas the
asthenosphere also transfers heat by convection.
•The key principle of plate tectonics is that the lithosphere exists as
separate and distinct tectonic plates, which ride on the fluid-like (
visco-elastic solid) asthenosphere. Plate motions range up to a
typical 10–40 mm/a (Mid-Atlantic Ridge; about as fast as
fingernails grow), to about 160 mm/a (Nazca Plate; about as fast as
hair grows).

8. Plate Tectonics
•Plate tectonics (from the Late Latin
tectonicus) is a scientific theory which
describes the large scale motions of Earth's
lithosphere.
•The theory builds on the older concepts of
continental drift, developed during the first
decades of the 20th century by
Alfred Wegener, and seafloor spreading,
developed in the 1960s.

9. Development of the theory
• geosynclinal theory
• Plate tectonic theory arose out of the hypothesis of
continental drift proposed by Alfred Wegener in 1912. He
suggested that the present continents once formed a single
land mass that drifted apart, thus releasing the continents
from the Earth's core and likening them to "icebergs" of low
density granite floating on a sea of denser basalt.
•Seafloor Spreading
The first evidence that the lithospheric plates did move came
with the discovery of variable magnetic field direction in rocks
of differing ages.

10. Seafloor Spreading Theory
•A profound consequence of seafloor spreading is that
new crust was, and is now, being continually created
along the oceanic ridges. This idea found great favor with
some scientists, most notably S. Warren Carey, who
claimed that the shifting of the continents can be simply
explained by a large increase in size of the Earth since its
formation. However, this so-called "
Expanding Earth theory" hypothesis was unsatisfactory
because its supporters could offer no convincing
mechanism to produce a significant expansion of the
Earth. Certainly there is no evidence that the moon has
expanded in the past 3 billion years.

11. Explanation of magnetic striping and Sea
floor Spreading
•The discovery of magnetic striping and the stripes being
symmetrical around the crests of the mid-ocean ridges
suggested a relationship. In 1961, scientists began to
theorise that mid-ocean ridges mark structurally weak zones
where the ocean floor was being ripped in two lengthwise
along the ridge crest. New magma from deep within the
Earth rises easily through these weak zones and eventually
erupts along the crest of the ridges to create new
oceanic crust. This process, later called seafloor spreading,
operating over many millions of years continues to form
new ocean floor all across the 50,000 km-long system of
mid-ocean ridges.

12. Seafloor Spreading

13. Evidences of Seafloor Spreading
•at or near the crest of the ridge, the rocks are very
young, and they become progressively older away
from the ridge crest;
•the youngest rocks at the ridge crest always have
present-day (normal) polarity;
•stripes of rock parallel to the ridge crest alternated in
magnetic polarity (normal-reversed-normal, etc.),
suggesting that the Earth's magnetic field has reversed
many times

14. Driving Forces of Plate Motion
•Tectonic plates are able to move because the Earth's
lithosphere has a higher strength and lower density
than the underlying asthenosphere. Their movement
is driven by heat dissipation from the mantle. Lateral
density variations in the mantle result in convection,
which is transferred into tectonic plate motion
through some combination of drag, downward suction
at the subduction zones, and variations in topography
and density of the crust that result in differences in
gravitational forces.

15. What Drives a Plate: Convection

16. Types of Plate Boundaries
•Three types of plate boundaries exist, characterized by the way the plates move
relative to each other. They are associated with different types of surface
phenomena. The different types of plate boundaries are:
•Transform boundaries occur where plates slide or, perhaps more accurately, grind
past each other along transform faults. The relative motion of the two plates is
either sinistral (left side toward the observer) or dextral (right side toward the
observer). The San Andreas Fault in California is an example of a transform
boundary exhibiting dextral motion.
•Divergent boundaries occur where two plates slide apart from each other. Mid-
ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's
Great Rift Valley) are both examples of divergent boundaries.
•Convergent boundaries (or active margins) occur where two plates slide towards
each other commonly forming either a subduction zone (if one plate moves
underneath the other) or a continental collision (if the two plates contain
continental crust). Deep marine trenches are typically associated with subduction
zones. The subducting slab contains many hydrous minerals, which release their
water on heating; this water then causes the mantle to melt, producing volcanism.
Examples of this are the Andes mountain range in South America and the Japanese
island arc.

17. Divergent Boundaries
•At divergent boundaries new crust is created as one or more
plates pull away from each other. Oceans are born and grow
wider where plates diverge or pull apart. As seen below, when
a diverging boundary occurs on land a 'rift', or separation will
arise and over time that mass of land will break apart into
distinct land masses and the surrounding water will fill the
space between them.
•Iceland is splitting along the Mid-Atlantic Ridge - a divergent
boundary between the North American and Eurasian Plates. As
North America moves westward and Eurasia eastward, new
crust is created on both sides of the diverging boundary.

18. Divergent Boundaries

19. Divergent Plate Boundary

20. Convergent Boundaries
•Crust is destroyed and recycled back into the interior of the Earth as one
plate dives under another. These are known as Subduction Zones -
mountains and volcanoes are often found where plates converge. There
are 3 types of convergent boundaries: Oceanic-Continental Convergence;
Oceanic-Oceanic Convergence; and Continental-Continental
Convergence.
•When an oceanic plate pushes into and subducts under a continental
plate, the overriding continental plate is lifted up and a mountain range
is created. Even though the oceanic plate as a whole sinks smoothly and
continuously into the subduction trench, the deepest part of the
subducting plate breaks into smaller pieces.
•These smaller pieces become locked in place for long periods of time
before moving suddenly generating large earthquakes. Such earthquakes
are often accompanied by uplift of the land by as much as a few meters.

21. Convergent Boundaries

22. Oceanic-Continental Convergence
•When a thin, dense oceanic plate collides with
a relatively light, thick continental plate, the
oceanic plate is forced under the continental
plate; this phenomenon is called subduction.

23. Subduction
•How can new crust be continuously added along the oceanic ridges without
increasing the size of the Earth?
•This question particularly intrigued Harry Hess, a Princeton University geologist
and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S.
Coast and Geodetic Survey who first coined the term seafloor spreading. Dietz
and Hess were among the small handful who really understood the broad
implications of sea floor spreading. If the Earth's crust was expanding along the
oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested
that new oceanic crust continuously spreads away from the ridges in a
conveyor belt-like motion. Many millions of years later, the oceanic crust
eventually descends into the oceanic trenches — very deep, narrow canyons
along the rim of the Pacific Ocean basin. Hess' ideas neatly explained why the
Earth does not get bigger with sea floor spreading, why there is so little
sediment accumulation on the ocean floor, and why oceanic rocks are much
younger than continental rocks.

24. Ocean-continent convergence

25. Block diagram of an ocean-continent convergent boundary. Oceanic lithosphere moves from left to right and
is subducted beneath the overriding continental lithosphere. Magma is created by partial melting of the
asthenosphere.

26. SUBDUCTION ZONE

27. Oceanic-Continental Plate Convergence

28. Oceanic-Oceanic Convergence
•When two oceanic plates collide, one may be pushed under the
other and magma from the mantle rises, forming volcanoes in the
vicinity. When two oceanic plates converge one is usually
subducted under the other and in the process a deep oceanic
trench is formed. The Marianas Trench, for example, is a deep
trench created as the result of the Phillipine Plate subducting
under the Pacific Plate.
•Oceanic-oceanic plate convergence also results in the formation
of undersea volcanoes. Over millions of years, however, the
erupted lava and volcanic debris pile up on the ocean floor until a
submarine volcano rises above sea level to form an island
volcano. Such volcanoes are typically streched out in chains
called island arcs.

29. A volcanic island arc forms as a result of oceanic-oceanic plate convergence.

30. Continent-continent collision
Continent-continent convergence is preceded by the closing of an ocean basin while ocean-
continent convergence takes place. Figure at the right shows the position of India relative to
the Eurasian plate in time. The convergence of the two plates created the Himalaya.

31. Continental-Continental Convergence
•When two continents meet head-on, neither is subducted
because the continental rocks are relatively light and, like two
colliding icebergs, resist downward motion. Instead, the crust
tends to buckle and be pushed upward or sideways. As a result
of two continental plates collision, mountain ranges are created as
the colliding crust is compressed and pushed upwards. The collision
of India into Asia 50 million years ago caused the Eurasian Plate
to crumple up and override the Indian Plate. After the collision,
the slow continuous convergence of the two plates over
millions of years pushed up the Himalayas and the Tibetan
Plateau to their present heights. Most of this growth occurred
during the past 10 million years.

32. Continental-Continental Convergence

33. Continent-continent collision
Continent-continent convergence is preceded by the closing of an ocean basin while ocean-
continent convergence takes place. Figure at the right shows the position of India relative to
the Eurasian plate in time. The convergence of the two plates created the Himalaya.

34. Transform-Fault Boundaries
•Transform-Fault Boundaries are where two plates are sliding horizontally past one
another. These are also known as transform boundaries or more commonly as
faults.
•When two plates move sideways against each other (at a transform plate
boundary), there is a tremendous amount of friction which makes the movement
jerky. The plates slip, then stick as the friction and pressure build up to incredible
levels. When the pressure is released suddenly, and the plates suddenly jerk
apart, this cause an earthquake.
•Most transform faults are found on the ocean floor. They commonly offset active
spreading ridges, producing zig-zag plate margins, and are generally defined by
shallow earthquakes. A few, however, occur on land.
•The San Andreas fault zone in California is a transform fault that connects the East
Pacific Rise, a divergent boundary to the south, with the South Gorda -- Juan de
Fuca -- Explorer Ridge, another divergent boundary to the north. The San Andreas
is one of the few transform faults exposed on land.
The San Andreas fault zone, which is about 1,300 km long and in places tens of
kilometers wide, slices through two thirds of the length of California. Along it, the
Pacific Plate has been grinding horizontally past the North American Plate for 10
million years, at an average rate of about 5 cm/yr. Land on the west side of the
fault zone (on the Pacific Plate) is moving in a northwesterly direction relative to
the land on the east side of the fault zone (on the North American Plate).

35. Transform-Fault Boundaries

36. Types of Plate Movements

37. Significance of Plate Tectonics
•The theory of plate tectonics (meaning "plate
structure") was developed in the 1960's. This
theory explains the movement of the Earth's
plates (which has since been documented
scientifically) and also explains the cause of
earthquakes, volcanoes, oceanic trenches,
mountain range formation, and many other
geologic phenomenon.

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